Seamless steel pipe and method of manufacturing the same

ABSTRACT

A seamless steel pipe contains, (mass %), C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, where carbon equivalent is not less than 0.430% and less than 0.500%, the microstructure main phase from the surface to an in-the-wall portion is tempered martensite or tempered bainite, prior austenite grain size is lower than 6.0, a portion between 1 mm from the inner surface and 1 mm from the outer surface has Vickers hardness of 250 Hv or lower, and yield strength is 555 MPa or higher.

TECHNICAL FIELD

The present invention relates to a seamless steel pipe and a method of manufacturing the same, and, more particularly, to a seamless steel pipe suitable for line pipe and a method of manufacturing the same.

BACKGROUND ART

Oil and gas resources from oil wells located on land and in shallow seas are drying up and, to address this, increasing numbers of offshore oil fields in deep seas are being developed. In an offshore oil field, crude oil or gas must be transported from the pithead of an oil well or gas well installed on the seabed to the platform above the sea using a flow line or a riser. A flow line is a line pipe laid along the topography of the surface of the earth or the seabed. A riser is a line pipe disposed to rise from the seabed toward the platform (i.e. upward).

The inner side of a steel pipe forming part of a flow line laid in the deep sea is subject to a high interior fluid pressure having a pressure from deep strata added thereto and, when the operation is halted, is also affected by seawater pressures of the deep sea. A steel pipe forming part of a riser is further affected by repeated distortions by ocean waves. Accordingly, it is desirable that steel pipes used for such applications have high strength and high toughness. In addition, oil and gas wells are being developed in sour environments, which are harsher than conditions for conventional wells, such as deep seas and cold regions. Offshore pipe lines laid in such harsh sour environments are required to have a higher strength (i.e. pressure resistance) and toughness than conventional ones, and are further required to have hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSC resistance).

Patent Document 1 discloses a seamless steel pipe with a large wall thickness for line pipe having high strength and good toughness, containing C: 0.03 to 0.08%, Si: 0.15 or less, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008% and one or more of Ca, Mg and REM: 0.0002 to 0.005% in total, the balance being Fe and impurities, where P in the impurities: 0.05% or less, S: 0.005% or less, and the wall thickness is 30 to 50 mm.

Patent Document 2 discloses a high-strength seamless steel pipe with a large wall thickness that is made by quenching and tempering and having a yield strength higher than 450 MPa for line pipe with good sour resistance where the Vickers hardness HV5 measurable at an outermost or innermost position of the pipe with an applied load of 5 kgf (with a force in the test of 49 N) is 250 HV5 or lower.

Patent Document 3 discloses a seamless steel pipe for line pipe containing, in mass %, C: 0.02 to 0.10%, Si: 0.5% or less, Mn: 0.5 to 2.0%, Al: 0.01 to 0.1%, Ca: 0.005% or less, and N: 0.007% or less, and one or more selected from the group consisting of Ti: 0.008% or less, V: less than 0.06% and Nb: 0.05% or less, the balance being Fe and impurities, where the total content of Ti, V and Nb is smaller than 0.06%, the carbon equivalent Ceq defined by the following equation is 0.38% or more, and the size of carbonitride particles containing one or more of Ti, V, Nb and Al is 200 nm or less.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

Patent Document 4 discloses a seamless steel pipe with a chemical composition of, in mass %, C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Mo: 0.5 to 1.0%, Cr: 0.1 to 1.0%, Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.005% or less, Ca: 0.0005 to 0.005%, V: 0.010 to 0.040%, and N: 0.002 to 0.007% and one or more selected from the group consisting of Ti: 0.001 to 0.008% and Nb: 0.02 to 0.05%, the balance being Fe and impurities, where the carbon equivalent Ceq is 0.50 to 0.58%, the pipe containing a specified carbide.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] JP 2010-242222 A

[Patent Document 2] JP 2013.32584 A

[Patent Document 3] WO 2011/152240

[Patent Document 4] JP 5516831 B

DISCLOSURE OF THE INVENTION

Even when one or more of the above conventional techniques are used, a seamless steel pipe having a strength of ×80 grade or higher as defined by the American Petroleum Institute (API) standards (i.e. a lower limit yield strength of 555 MPa or higher) may not have good SSC resistance in a reliable manner.

To improve the strength and toughness of a seamless steel pipe produced by quenching-and-tempering, the content of alloy elements such as carbon may be increased to increase hardenability. However, if the content of alloy elements such as carbon is increased, the strength (i.e. hardness) of the surface of the steel pipe increases. In a seamless steel pipe produced by quenching-and-tempering, the surface layer is cooled at a high rate during quenching and can easily be hardened, increasing the hardness, while the in-the-wall portions have low hardness. This tendency may remain after tempering. As such, in a seamless steel pipe having a strength of ×80 grade or higher, a surface layer hardness may exceed 250 Hv, which is the higher limit required in the sour resistance grade according to the API 5 L standards.

Although the techniques of Patent Document 1 are effective in achieving high strength and high toughness, they do not sufficiently consider reducing the hardness of the surface layer or thus improving SSC resistance. Patent Document 2 states that the hardness of the surface layer of a steel pipe can be controlled to be 250 HV5 or lower; however, it appears to require a special manufacturing process. Patent Document 3 provides some considerations about SSC resistance; however, after hot forming, it is necessary to perform direct quenching or in-line quenching and then reheating-and-quenching. Patent Document 4 provides some considerations about the hardness of the surface layer of a steel pipe and HIC resistance; however, a reheating-and-quenching step is necessary and, after hot forming, direct quenching or in-line quenching is used as necessary, which means manufacturing costs that are not very reasonable.

An object of the present invention is to provide a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

A seamless steel in an embodiment of the present invention has a chemical composition of, in mass %, C: 0.02 to 0.15% Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities, where a carbon equivalent Ceq as defined by equation (1) below is not less than 0.430% and less than 0.500%, a main phase of a microstructure from a surface layer to an in-the-wall portion is tempered martensite or tempered bainite, a size of prior austenite grains in the microstructure is lower than 6.0 in crystal grain size number according to ASTM E112-10, a portion between a position at 1 mm from an inner surface and a position at 1 mm from an outer surface has a Vickers hardness of 250 Hv or lower, and a yield strength is 555 MPa or higher,

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (1),

where a symbol of each element in equation (1) is substituted by a content of this element in mass %.

A method of manufacturing a seamless steel pipe in an embodiment of the present invention includes: preparing a raw material having a chemical composition of, in mass %, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities; hot working the raw material to produce a hollow shell; quenching the hollow shell by direct quenching or in-line quenching; and tempering the quenched hollow shell. No reheating-and-quenching is performed between the quenching and tempering. A carbon equivalent Ceq as defined by equation (3) below is not less than 0.430% and less than 0.500%, a Larson-Miller parameter PL as defined by equation (4) below is not less than 18800,

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (3), and

PL=(T+273)×(20+log(t))  (4).

A symbol of each element in equation (3) is substituted by a content of this element in mass %. In equation (4), T is a tempering temperature, and t is a holding period for this temperature. T is in ° C., and t is in hours.

The present invention provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a manufacturing line.

FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe.

FIG. 3 shows changes in the surface temperature of a workpiece during a manufacture versus time.

FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel. B.

FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A.

FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B.

FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A.

FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B.

FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present inventors did research to find a method of providing a seamless steel pipe that ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner. They found out that limiting the carbon equivalent of a steel to an appropriate range and reducing the difference between the hardness of the surface layer and the hardness of the in-the-wall portions of the seamless steel pipe ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner, where only direct quenching or in-line quenching is performed after hot forming and no reheating-and-quenching is performed.

During the quenching after rolling, the surface layer of a seamless steel pipe is cooled at high rate and can easily be hardened. As such, the surface layer of the seamless steel pipe tends to be hard and may exceed the values of hardness specified by the API 5 L standards or DNV-OS-F101 standards. On the other hand, the portions located in the middle in the wall thickness of the seamless steel pipe is cooled at a lower rate and cannot easily be hardened such that non-quenched structures such as ferrite may be included. Thus, there is typically a difference between the hardness of the surface layer and that of the in-the-wall portions, and this tendency may remain after tempering for certain tempering conditions. Further, in a seamless steel pipe with high carbon equivalent such as those used in high-strength steel with ×80 grade or higher, the difference between the hardness of the surface layer and that of the in-the-wall portions tends to be significant. Such a high hardness of the surface layer may be a problem when good sour resistance is to be achieved in a reliable manner.

If the carbon equivalent is too low, it is difficult to ensure a certain strength of a seamless steel pipe. If the carbon equivalent is too high, it is difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower with a manufacturing process in which reheating-and-quenching is eliminated, direct quenching or in-line quenching being only one step of the quenching. This is because, if the quenching after hot forming is direct quenching or in-line quenching, the austenite grains tend to be coarse compared with implementations where reheating-and-quenching is performed, which increases overall hardenability. In view of this, Ceq as defined by equation (1) below is to be not less than 0.430% and less than 0.500%:

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (1),

where the symbol of each element in equation (1) is substituted by the content of this element in mass %.

To reduce the difference between the hardness of the surface layer and that of the in-the-wall portions, it is effective to limit the carbon equivalent and, in addition, the tempering conditions appropriately. That is, if tempering is not sufficiently done, the reduction in the hardness of the surface layer is insufficient such that some portions may have a Vickers hardness higher than 250 Hv. More specifically, the Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher.

PL=(T+273)×(20+log(t))  (2).

In equation (2), T is a tempering temperature (in ° C.) and t is a holding time (in hours) for that temperature.

The present invention was made based on the above findings. A seamless steel pipe in one embodiment of the present invention will now be described in detail with reference to the drawings. The same or corresponding portions in the drawings are labeled with the same characters and their description will not be repeated.

[Chemical Composition]

The seamless steel pipe in the present embodiment has the chemical composition described below. In the following description, “%” for the content of an element means mass %.

C: 0.02 to 0.15%

Carbon (C) increases the strength of the steel. If the C content is lower than 0.02%, this effect cannot be sufficiently achieved. If the C content is higher than 0.15%, the toughness of the steel decreases. In view of this, the C content should be in the range of 0.02 to 0.15%. The C content is preferably higher than 0.02%, and more preferably 0.04% or higher. The C content is preferably 0.10% or lower, and more preferably 0.08% or lower.

Si: 0.05 to 0.5%

Silicon (Si) deoxidizes steel. This effect can be clearly achieved if the Si content is 0.05% or higher. However, if the Si content is higher than 0.5%, the toughness of the steel decreases. In view of this, the Si content should be in the range of 0.05 to 0.5%. The Si content is preferably higher than 0.05%, and more preferably 0.08% or higher, and still more preferably 0.10% or higher. The Si content is preferably lower than 0.5%, and more preferably 0.25% or lower, and still more preferably 0.20% or lower.

Mn: 0.30 to 2.5%

Manganese (Mn) increases the hardenability of steel to increase the strength of the steel. These effects cannot be sufficiently achieved if the Mn content is lower than 0.30%. If the Mn content is higher than 2.5%, Mn segregates in the steel, decreasing the toughness of the steel. In view of this, the Mn content should be in the range of 0.30 to 2.5%. The Mn content is preferably higher than 0.30%, and more preferably 1.0% or higher, and still more preferably 1.3% or higher. The Mn content is preferably lower than 2.5%, and more preferably 2.0% or lower, and still more preferably 1.8% or lower.

P: Up to 0.03%

Phosphorus (P) is an impurity. P decreases the toughness of steel. Thus, lower P contents are preferable. In view of this, the P content should be 0.03% or lower. The P content is preferably lower than 0.03%, and more preferably 0.015% or lower, and still more preferably 0.012% or lower.

S: Up to 0.006%

Sulphur (S) is an impurity. S bonds with Mn to form coarse MnS particles and thus decreases the toughness and HIC resistance of the steel. Thus, lower S contents are preferable. In view of this, the S content should be 0.006% or lower. The S content is preferably lower than 0.006%, and more preferably 0.003% or lower, and still more preferably 0.002% or lower.

O: Up to 0.004%

Oxygen (O) is an impurity. O forms coarse oxide particles or clusters of oxide particles, decreasing the toughness of the steel. Thus, lower O contents are preferable. In view of this, the O content should be 0.004% or lower. The O content is preferably 0.003% or lower, and more preferably 0.002% or lower.

Al: 0.01 to 0.10%

Aluminum (Al) bonds with N to form fine nitride particles, increasing the toughness of the steel. This effect cannot be sufficiently achieved if the Al content is lower than 0.01%. If the Al content is higher than 0.10%, coarse Al nitride particles result, decreasing the toughness of the steel. In view of this, the Al content should be in the range of 0.01 to 0.10%. The Al content is preferably higher than 0.01%, and more preferably 0.02% or higher. The Al content is preferably lower than 0.10%, and more preferably 0.08% or lower, and still more preferably 0.06% or lower. As used herein, Al content means the content of acid-soluble Al (i.e. so-called “sol. Al”).

Ti: 0.001 to 0.010%

Titanium (Ti) bonds with N in a steel and forms TiN, suppressing the reduction in the toughness of the steel due to dissolved N. Further, the dispersed and precipitated fine TiN particles increase the toughness of the steel. These effects cannot be sufficiently achieved if the Ti content is lower than 0.001%. If the Ti content is higher than 0.010%, coarse TiN particles result or coarse TiC particles are produced, decreasing the toughness of the steel. In view of this, the Ti content should be in the range of 0.001 to 0.010%. The Ti content is preferably higher than 0.001% and more preferably 0.002% or higher. The Ti content is preferably lower than 0.010%, and more preferably 0.006% or lower, and still more preferably 0.005% or lower.

N: Up to 0.007%

Nitrogen (N) bonds with Al and forms fine Al nitride particles, increasing the toughness of the steel. However, if the N content is higher than 0.007%, dissolved N decreases the toughness of the steel. Further, if the N content is too high, coarse carbonitride and/or nitride particles result, decreasing the toughness of the steel. In view of this, the N content should be 0.007% or lower. The N content is preferably lower than 0.007%, and more preferably 0.006% or lower, and still more preferably 0.005% or lower. The N content is preferably 0.002% or higher.

Cr: 0.05 to 1.0%

Chromium (Cr) increases the hardenability of steel and increases the strength of the steel. Cr further increases the temper softening resistance of the steel. These effects cannot be sufficiently achieved if the Cr content is lower than 0.05%. If the Cr content is higher than 1.0%, the toughness of the steel decreases. In view of this, the Cr content should be in the range of 0.05 to 1.0%. The Cr content is preferably higher than 0.05%, and more preferably 0.2% or higher. The Cr content is preferably lower than 1.0%, and more preferably 0.8% or lower.

Mo: Not Less than 0.02% and Less than 0.5%

Molybdenum (Mo) improves the strength of steel by transformation toughening and solute strengthening. This effect cannot be sufficiently achieved if the Mo content is lower than 0.02%. If the Mo content is higher than 0.5%, the toughness of the steel decreases. In view of this, the Mo content should be not lower than 0.02% and lower than 0.5%. The Mo content is preferably higher than 0.02%, and more preferably 0.05% or higher, and still more preferably 0.1% or higher. The Mo content is preferably 0.4% or lower, and more preferably 0.3% or lower.

Ni: 0.03 to 1.0%

Nickel (Ni) increases the hardenability of steel and increases the strength of the steel. Further, Ni has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Ni content is lower than 0.03%. If the Ni content is higher than 1.0%, the SSC resistance decreases. In view of this, the Ni content should be in the range of 0.03 to 1.0%. The Ni content is preferably 0.05% or higher, and more preferably 0.08% or higher, and still more preferably 0.10% or higher. The Ni content is preferably lower than 1.0%, and more preferably 0.7% or lower, and still more preferably 0.5% or lower.

Cu: 0.02 to 1.0%

Copper (Cu) increases the hardenability of steel and increases the strength of the steel. Further, Cu has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Cu content is lower than 0.02%. If the Cu content is higher than 1.0%, the weldability of the steel decreases. Further, if the Cu content is too high, the grain boundary strength of the steel at high temperatures decreases, decreasing the hot workability of the steel. In view of this, the Cu content should be in the range of 0.02 to 1.0%. The Cu content is preferably 0.05% or higher, and more preferably 0.08% or higher, and still more preferably 0.10% or higher. The Cu content is preferably lower than 1.0%, and more preferably 0.7% or lower, and still more preferably 0.5% or lower.

V: 0.020 to 0.20%

Vanadium (V) bonds with C in a steel and forms a V carbide to increase the strength of the steel. Further, V is dissolved in an Mo carbide to form a carbide. A carbide containing V is less likely to form coarse particles. These effects cannot be effectively achieved if the V content is lower than 0.020%. If the V content is higher than 0.20%, coarse carbide particles result. In view of this, the V content should be in the range of 0.020 to 0.20%. The V content is preferably higher than 0.020%, and more preferably 0.04% or higher. The V content is preferably lower than 0.16%.

Ca: 0.0005 to 0.005%

Calcium (Ca) bonds with S in steel to form CaS. As CaS is formed, the formation of MnS is suppressed. Thus, Ca increases the toughness and HIC resistance of the steel. These effects cannot be sufficiently achieved if the Ca content is lower than 0.0005%. If the Ca content is higher than 0.005%, the cleanliness of the steel decreases, decreasing the toughness and HIC resistance of the steel. Thus, the Ca content should be in the range of 0.0005 to 0.005%. The Ca content is preferably higher than 0.0005%, and more preferably 0.0008% or higher, and still more preferably 0.001% or higher. The Ca content is preferably lower than 0.005%, and more preferably 0.003% or lower, and still more preferably 0.002% or lower.

The balance of the chemical composition of the seamless steel pipe in the present embodiment is made of Fe and impurities. Impurity in this context means an element originating from ore or scraps used as a raw material of steel or an element that has entered from the environment or the like during the manufacturing process.

Further, the chemical composition of the seamless steel pipe in the present embodiment may contain Nb in lieu of some of Fe.

Nb: 0 to 0.05%

Niobium (Nb) is an optional element. Nb bonds with C and/or N in steel and forms fine Nb carbide and/or carbonitride particles to increase the toughness of the steel. Further, Nb is dissolved in an Mo carbide and forms a specified carbide, thereby preventing coarse particles of a specified carbide from being produced. On the other hand, if the Nb content is higher than 0.05%, coarse carbide particles result. In view of this, the Nb content should be in the range of 0 to 0.05%. The above effects can be clearly achieved if the Nb content is 0.010% or higher. The Nb content is preferably 0.015% or higher, and more preferably 0.020% or higher. The Nb content is preferably 0.040% or lower, and more preferably 0.035% or lower.

[Carbon Equivalent Ceq]

In the seamless steel pipe in the present embodiment, a carbon equivalent Ceq as defined by equation (1) is not less than 0.430% and less than 0.500%.

Ceq=C+Mn/6+(Cr+Mo+V)/5+F(Ni+Cu)/15  (1),

where the symbol of each element in equation (1) is substituted by the content of this element in mass %.

If the carbon equivalent Ceq is lower than 0.430%, it is difficult to ensure a certain strength of a seamless steel pipe. If the carbon equivalent Ceq is 0.500 or higher, it is difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching.

[Microstructure]

In the microstructure of the seamless steel pipe in the present embodiment, the main phase from the surface layer to the in-the-wall portions is tempered martensite or tempered bainite. The seamless steel pipe in the present embodiment contains no recrystallized ferrite at least in a region deeper than a position 1 mm deep relative to the surface. Recrystallized ferrite extremely reduces the hardness of a portion at 1 mm from the surface layer of the seamless steel pipe.

The main phase being tempered martensite or tempered bainite generally means a microstructure in which the volume fraction of tempered martensite is 50% or higher, a microstructure in which the volume fraction of tempered bainite is 50% or higher, or a microstructure in which the sum of the volume fraction of tempered martensite and the volume fraction of tempered bainite is 50% or higher. In other words, the above phrase means a microstructure in which the volume fraction of a structure that is neither tempered martensite nor tempered bainite (for example, ferrite) is lower than 50%.

[Crystal Grain Size Number]

In the microstructure of the seamless steel pipe of the present embodiment, the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E112-10.

The prior austenite grain size number may be measured in accordance with ASTM E112-10 by cutting out a test specimen from each steel pipe preferably before tempering and after quenching, such that a cross section perpendicular to the length of the steel pipe (i.e. pipe forming direction) forms the observed surface, and imbedding the test specimen into a resin and then using the Bechet-Beaujard method where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear.

Alternatively, the ASTM grain size number of prior austenite crystal grains of the tempered steel pipe may be determined by using methods such as electron beam backward scattering diffraction (EBSD) based on the orientation relationship of crystals. In such cases, the metal microstructure of a steel pipe after tempering is observed by EBSD in the following manner: A sample is obtained from the middle in the wall thickness in a cross section of a tempered seamless steel pipe (i.e. cross section perpendicular to the axial direction of the seamless steel pipe); the obtained sample is used to perform crystal orientation analysis by EBSD for an observed area of 500×500 pmt, and lines are drawn where a prior austenite grain boundary is defined as the boundary of grains in a misorientation angle in the range of 15 to 51° and, based on the resulting drawing, the crystal grain size number is calculated in accordance with ASTM E112-10.

Theoretically, the prior austenite grain size after quenching and before tempering is the same as the prior austenite grain size after tempering. The prior austenite grain size determined by EBSD after tempering is substantially equal to the value obtained by observing crystal grains that were caused to appear by the Bechet-Beaujard method after quenching and before tempering, with an error of about ±0.2 in grain size number. Thus, “the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E112-10” as in the present invention means that, if the crystal grain size after quenching is not known, at least, a crystal grain size number determined by EBSD after tempering being lower than 5.8 is in the scope of the present invention. In the following description, unless specifically stated, prior austenite grain size is a value obtained by the Bechet-Beaujard method for a test specimen after quenching and before tempering.

If the prior austenite grains are fine grains with a crystal grain size number of 6.0 or higher, sufficient hardenability cannot be achieved in a material with a low carbon equivalent Ceq, as in the present embodiment. Thus, a predetermined strength may not be obtained. Further, it is difficult to produce a microstructure with such fine grains with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching. The crystal grain size number of prior austenite grains is preferably 5.5 or lower, and more preferably, 5.0 or lower.

[Vickers Hardness and Yield Strength]

In the seamless steel pipe in the present embodiment, a portion between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface has a Vickers hardness of 250 Hv or lower. More specifically, in the seamless steel pipe in the present embodiment, the Vickers hardness measured in compliance with JIS Z 2244 at any position between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface is 250 Hv or lower.

The seamless steel pipe of the present invention has smaller variations in hardness along the wall thickness direction. More specifically, the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in the middle in the wall thickness, the difference between the Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface is 25 Hv or lower.

The seamless steel pipe in the present embodiment has a yield strength of ×80 grade or higher (i.e. 555 MPa or higher) according to the API standards.

The seamless steel pipe in the present embodiment may be suitably used as, although not limited thereto, a seamless steel pipe with a wall thickness of 25 to 55 mm. More preferably, to rationalize the use of alloys, the wall thickness of a seamless steel pipe is in the range of 25 to 40 mm.

[Manufacturing Method]

An example of a method of manufacturing the seamless steel pipe in the present embodiment will be described below. However, the method of manufacturing the seamless steel pipe in the present embodiment is not limited thereto.

[Manufacturing Line]

FIG. 1 is a block diagram illustrating an example of a manufacturing line. Referring to FIG. 1, the manufacturing line includes a heating furnace 1, a piercing machine 2, an elongation rolling mill 3, a sizing rolling mill 4, a supplementary heating furnace 5, a water-cooling apparatus 6, and a tempering apparatus 7. A plurality of transport rollers 10 are disposed between these apparatuses.

[Manufacturing Flow]

FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe in the present embodiment. FIG. 3 shows changes in the surface temperature of a workpiece (i.e. a steel raw material, hollow shell or seamless steel pipe) during a manufacture versus time. In the graph, A1 indicates the Ac₁ point when considering a workpiece being heated, and indicates the Ar₁ point when considering a workpiece being cooled. Further, in the graph, A3 indicates the Ac₃ point when considering a workpiece being heated, and indicates the Ar₃ point when considering a workpiece being cooled.

As shown in FIGS. 1 to 3, the manufacturing process involves first heating a steel raw material using the heating furnace 1 (heating step: S1). The steel raw material may be a round billet, for example. The steel raw material may be produced by a continuous casting system such as round CC. The steel raw material may be produced by hot working (e.g. forging or blooming) an ingot or slab. A case with a steel raw material that is a round billet will be described below.

The heated round billet is hot-worked to produce a seamless steel pipe (S2 and S3). More specifically, the round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell (piercing-rolling step: S2). Further, the hollow shell is rolled by the elongation rolling mill 3 and sizing rolling mill 4 to produce a seamless steel pipe (elongation rolling step and sizing rolling step S3).

The seamless steel pipe produced by the hot working is heated to a predetermined temperature by the supplementary heating furnace 5 as necessary (supplementary heating step: S4). The seamless steel pipe produced by the hot working or the heated seamless steel pipe is quenched by the water-cooling apparatus 6 (quenching step: S5). In either case, the seamless steel pipe produced by the hot working is quenched without being cooled to lower than Ar₃ temperature. The quenched seamless steel pipe is tempered by the tempering apparatus 7 (tempering step S6).

That is, in the above manufacturing method, quenching is performed promptly after the hot working is finished. More specifically, after hot working, quenching is performed before the temperature of the seamless steel pipe is left to cool to decrease to around room temperature. A heat treatment where a seamless steel pipe after hot working is rapidly cooled before the surface temperature becomes lower than the Ar₃ point will be hereinafter referred to as “direct quenching”, and a heat treatment where a seamless steel pipe after hot working is supplementarily heated at a temperature not lower than the Ac₃ point and then rapidly cooled will be hereinafter referred to as “in-line quenching”. The use of direct quenching or in-line quenching makes the grains of the microstructure coarser than with a heat treatment in which a pipe is cooled after its production and then rapidly cooled (hereinafter referred to as reheating-and-quenching). More specifically, the crystal grain size number after quenching is smaller than 6.0. This improves the hardenability of a microstructure compared with the reheating-and-quenching, and thus ensures a high strength even when a steel material with a low carbon equivalent Ceq is used.

The steps will be described in more detail below.

[Heating Step (S1)]

A round billet is heated in the heating furnace 1. The heating temperature is preferably in the range of 1100 to 1300° C. Heating the round billet to this temperature range causes the carbonitride in the steel to dissolve. If a round billet is to be produced from a slab or ingot by hot working, it is only required that the slab or ingot be heated to a temperature of 1100 to 1300° C., and the temperature to which the round billet is heated by the heating furnace 1 does not have to be in the range of 1100 to 1300° C., because the carbonitride in the steel dissolves when the ingot or slab is being heated. The heating furnace 1 may be a walking-beam furnace or a rotary furnace, for example.

[Piercing Step (S2)]

The round billet is removed from the heating furnace 1 and the heated round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell. The piercing machine 2 includes a plurality of skewed rolls and a plug. The plug is disposed between the skewed rolls. Preferably, the piercing machine 2 is a cross-type piercer. A cross-type piercer is preferable because it can do piercing at high pipe expansion rate.

[Elongation Rolling Step and Sizing Rolling Step (S3)]

Next, the hollow shell is rolled. More specifically, the hollow shell is elongation-rolled by the elongation rolling mill 3. The elongation rolling mill 3 includes a plurality of roll stands disposed in series. The elongation rolling mill 3 may be a mandrel mill, for example. Subsequently, the hollow shell that has been subjected to elongation rolling is subjected to reduction rolling by the sizing rolling mill 4 to produce a seamless steel pipe. The sizing rolling mill 4 includes a plurality of roll stands disposed in series. The sizing rolling mill 4 may be a sizer or stretch reducer, for example. The elongation rolling step and sizing rolling step together may be referred to simply as rolling step.

[Supplementary Heating Step (S4)]

The supplementary heating step (S4) is performed as necessary. That is, the manufacturing method in the present embodiment need not include the supplementary heating step (S4). More specifically, the supplementary heating step (S4) is performed in such a way that the temperature of the seamless steel pipe is at a predetermined level that is equal to or higher than the Acs point directly before the water cooling of the quenching step (S5). If the supplementary heating step (S4) is not performed, the method in FIG. 2 proceeds from step S3 to step S5. If the supplementary heating step (S4) is not performed, the supplementary heating furnace 5 in FIG. 1 may not be provided.

If the finishing temperature of the rolling step (i.e. surface temperature of the seamless steel pipe directly after the rolling step is finished) is lower than 800° C., it is preferable to perform the supplementary heating step (S4). At the supplementary heating step (S4), the seamless steel pipe is inserted into the supplementary heating furnace 5 and heated. The heating temperature in the supplementary heating furnace 5 is preferably in the range of 900 to 1100° C. The soaking time is preferably 30 minutes or less. If the soaking time is too long, carbonitrides made of Ti, Nb, C and N, i.e. (Ti, Nb) and (C, N), may precipitate and form coarse particles. At the supplementary heating step, the supplementary heating furnace 5 may be replaced by an induction heating apparatus.

[Quenching Step (S5)]

The seamless steel pipe is water-cooled in the water-cooling apparatus 6. The temperature (i.e. surface temperature) of the seamless steel pipe directly before water cooling is equal to or higher than the Ac₃ point, and preferably equal to or higher than 800° C.

For water cooling, it is preferable that the cooling rate for the temperature range of the seamless steel pipe from 800° C. to 500° C. is equal to or higher than 5° C./sec (300° C./min). This provides a uniform quenched microstructure. The cooling is stopped at a temperature that is equal to or lower than the Ar₁ point. The temperature at which cooling is stopped is preferably 450° C. or lower, and the cooling may be done down to room temperature. The quenching step (S5) changes the structure of the matrix to a structure mainly composed of martensite or bainite.

For example, the water-cooling aperture 6 used for the quenching step (S5) may have the following construction: The water-cooling apparatus 6 includes a plurality of rotating rollers, laminar water flow device, and a jet water flow device. The rotating rollers are disposed in two rows, and the seamless steel pipe is positioned between the two rows of rotating rollers. At this time, the rotating rollers in the two rows are in contact with bottom portions of the outer surface of the seamless steel pipe. When the rotating rollers rotate, the seamless steel pipe rotates about its axis. The laminar water flow device is located above the rotating rollers and pours water from above the seamless steel pipe. At this time, the water poured toward the seamless steel pipe forms a laminar water flow. The jet water flow device is located near an end of the seamless steel pipe positioned on the rotating rollers. The jet water flow device emits a jet water flow from the end of the seamless steel pipe toward the interior of the steel pipe. The laminar and jet water flow devices cool the outer and inner surfaces of the seamless steel pipe at the same time. A water-cooling device 6 with such a construction is suitable for accelerated cooling for a seamless steel pipe with a large wall thickness of 25 mm or larger.

The water-cooling device 6 may be a device other than the one including rotating rollers, laminar water flow device and jet water flow device discussed above. The water cooling device 6 may be a water tank, for example. In such implementations, the seamless steel pipe is immersed in the water tank and thus subjected to accelerated cooling. Alternatively, the water-cooling device 6 may include a laminar water flow device only. To sum up, the cooling device 6 is not limited to a specific type.

[Tempering Step (S6)]

The quenched seamless steel pipe is subjected to tempering. More specifically, the quenched seamless steel pipe is heated to a predetermined tempering temperature that is lower than the Ac1 point, and is held at this temperature for a predetermined period of time in such a way that the Larson-Millar parameter PL as defined by equation (2) below is 18800 or higher:

PL=(T+273)×(20+log(t))  (2).

In equation (2), T is a tempering temperature (° C.), t is a holding time (in hours) for that temperature. Log (t) is the logarithm of t whose base is 10.

If PL is lower than 18800, the reduction in surface hardness is insufficient and some portions may have a Vickers hardness exceeding 250 Hv. PL is preferably 18900 or higher.

If PL is too high, the recrystallization of ferrite occurred in a region of a depth of 1 mm or deeper from the surface, which may cause an extreme reduction in strength, a reduction in the sour resistance in the surface layer and production of blisters. PL is preferably 20000 or lower, and more preferably 19500 or lower.

The lower limit of tempering temperature is preferably 600° C., and more preferably 630° C., and still more preferably 650° C. The upper limit of tempering temperature is preferably 700° C., and more preferably 680° C. The lower limit of holding time is preferably one hour, and more preferably two hours, and still more preferably three hours. The upper limit of holding time is preferably six hours, and more preferably five hours, and still more preferably four hours.

The above manufacturing process provides a seamless steel pipe with a wall thickness that is as large as 25 mm or more having good strength, toughness and HIC resistance. The above manufacturing method is particularly suitable for a seamless steel pipe with a wall thickness of 25 mm or larger, and can even be used for a seamless steel pipe with a wall thickness of 40 mm or larger. The upper limit of wall thickness is not limited to a specific value, but is typically 60 mm or lower.

The seamless steel pipe in one embodiment of the present invention and the method of manufacturing the same have been described. The present embodiment provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

EXAMPLES

The present invention will be described using specific examples. The present invention is not limited to these examples.

A plurality of seamless steel pipes with various chemical compositions were produced and their yield strength, tensile strength, surface hardness and sour resistance were investigated.

[Investigation Methods]

A plurality of steels having the chemical compositions shown in Table 1 were melt and were subjected to continuous casting to produce round billets for pipe forming. Steels A, C, D1, D2 and J in Table 1 are steels in which the chemical composition or the value of Ceq does not meet the requirements of the present invention.

TABLE 1 Chemical Composition (in mass %, balance being Fe and impurities) Steel C Si Mn P S Cu Cr Ni Mo Ti V Nb Al Ca O N Ceq A 0.059 0.12 1.53 0.005 0.0007 0.20 0.28 0.23 0.10 0.003 0.05 — 0.031 0.0008 0.0015 0.0036 0.429 B 0.061 0.11 1.51 0.008 0.0009 0.20 0.31 0.31 0.25 0.003 0.05 — 0.032 0.0017 0.0018 0.0050 0.468 C 0.070 0.09 1.42 0.011 0.0005 0.41 0.31 0.39 0.35 0.005 0.05 — 0.030 0.0013 0.0017 0.0048 0.502 D1 0.066 0.12 1.46 0.009 0.0010 0.02 0.23 0.08 0.09 0.010 0.05 — 0.037 0.0017 0.0018 0.0039 0.390 D2 0.065 0.12 1.44 0.009 0.0010 0.08 0.26 0.09 0.06 0.007 0.05 — 0.041 0.0016 0.0016 0.0041 0.390 E 0.068 0.11 1.51 0.009 0.0019 0.37 0.28 0.49 0.25 0.004 0.05 — 0.030 0.0012 0.0012 0.0032 0.493 F 0.061 0.11 1.51 0.010 0.0010 0.20 0.20 0.28 0.25 0.004 0.05 — 0.030 0.0014 0.0015 0.0043 0.445 G 0.060 0.12 1.52 0.009 0.0010 0.21 0.21 0.28 0.25 0.005 0.05 0.020 0.034 0.0012 0.0011 0.0050 0.448 I 0.062 0.12 1.52 0.005 0.0008 0.21 0.27 0.23 0.11 0.006 0.05 0.025 0.031 0.0008 0.0015 0.0036 0.431 J 0.061 0.11 1.42 0.011 0.0018 0.36 0.28 0.49 0.51 0.005 0.04 — 0.030 0.0013 0.0013 0.0032 0.520 K 0.058 0.12 1.50 0.008 0.0010 0.20 0.31 0.32 0.26 0.003 0.05 — 0.033 0.0016 0.0017 0.0055 0.467

The round billets produced were heated by the heating furnace to a temperature in the range of 1100 to 1300° C. Subsequently, the round billets were piercing-rolled by the piercing machine to produce hollow shells. Subsequently, the mandrel mill was used to elongation-roll the hollow shells. Subsequently, the sizer was used to reduction-roll (i.e. sizing-roll) the hollow shells to produce seamless steel pipes having the outer diameters and wall thicknesses shown in Tables 2 and 3.

TABLE 2 Pipe-Forming AsQ Mechanical Properties AsQ Conditions Prior Hv10kgf (maximum Prior Wall γ Tempering Conditions among positions) γ Outer Thick- grain Soaking Holding Inner Dif- grain Ferrite Diameter ness size Time Time YS TS Outer In Sur- fer- size Recrystal- No. Steel (mm) (mm) No. (° C.) (min) PL (MPa) (MPa) Surface Wall face ence No. lization Remarks 1 A 273.1 25.0 4.3 660 204 19156 518 592 202 198 208 10 4.2 absent comparative ex. 2 A 273.1 25.0 4.3 660 219 19185 501 577 212 200 213 13 4.3 absent comparative ex. 3 A 273.1 25.0 4.5 665 234 19314 509 580 197 195 162 35 4.4 present comparative ex. 4 A 273.1 25.0 4.5 650 204 18951 524 602 203 203 220 17 4.5 absent comparative ex. 5 A 273.1 25.0 4.3 650 219 18979 519 593 202 196 214 18 4.6 absent comparative ex. 6 A 273.1 25.0 4.3 650 234 19006 511 585 197 201 215 18 4.1 absent comparative ex. 7 A 273.1 25.0 4.3 650 249 19030 506 585 202 200 219 19 4.6 absent comparative ex. 8 A 273.1 25.0 4.3 650 264 19054 514 588 200 201 219 19 4.5 absent comparative ex. 9 A 273.1 25.0 4.3 650 294 19097 497 573 199 194 198 5 4.4 absent comparative ex. 10 A 273.1 25.0 4.3 650 205 18953 544 619 218 218 220 2 4.3 absent comparative ex. 11 A 273.1 25.0 4.3 630 204 18540 543 622 213 212 248 36 4.3 absent comparative ex. 12 A 273.1 25.0 4.3 630 219 18568 541 620 209 210 236 27 4.4 absent comparative ex. 13 A 273.1 25.0 4.3 630 234 18594 531 610 213 208 242 34 4.3 absent comparative ex. 14 A 273.1 25.0 4.5 630 249 18618 531 610 206 202 240 38 4.4 absent comparative ex. 15 A 273.1 25.0 4.5 630 264 18641 536 615 211 209 239 30 4.5 absent comparative ax. 16 A 273.1 25.0 4.3 630 294 18683 526 602 210 203 238 35 4.3 absent comparative ex. 17 A 273.1 25.0 4.3 600 204 17924 531 622 210 213 257 47 4.4 absent comparative ax. 18 B 323.9 25.0 4.3 700 294 20132 503 582 209 193 170 39 4.5 present comparative ex. 19 B 323.9 25.0 4.3 700 204 19977 575 641 192 193 209 17 4.3 absent inventive ex. 20 B 323.9 25.0 4.3 690 204 19772 578 646 203 206 211 8 4.4 absent inventive ex. 21 B 323.9 25.0 4.5 680 204 19566 579 646 212 220 222 10 4.5 absent inventive ex. 22 B 323.9 25.0 4.3 670 204 19361 597 659 236 220 239 19 4.2 absent inventive ex. 23 B 323.9 25.0 4.3 665 149 19131 621 688 238 240 239 2 4.3 absent inventive ex. 24 B 323.9 25.0 4.3 660 204 19156 606 670 225 226 239 14 4.4 absent inventive ex. 25 B 323.9 25.0 4.3 660 219 19185 601 665 224 230 239 15 4.3 absent inventive ex. 26 B 323.9 25.0 4.3 665 234 19314 600 664 233 226 235 9 4.3 absent inventive ex. 27 B 323.9 25.0 4.5 650 204 18951 631 697 248 244 249 5 4.5 absent inventive ex. 28 B 323.9 25.0 4.5 650 219 18979 620 683 235 235 248 13 4.7 absent inventive ex. 29 B 323.9 25.0 4.5 650 234 19006 620 681 235 235 248 13 4.5 absent inventive ex. 30 B 323.9 25.0 4.3 650 249 19030 617 683 242 226 248 22 4.3 absent inventive ex.

TABLE 3 Pipe-Forming AsQ Mechanical Properties AsQ Conditions Prior Hv10kgf (maximum Prior Wall γ Tempering Conditions among positions) γ Outer Thick- grain Soaking Holding Inner Dif- grain Ferrite Diameter ness size Time Time YS TS Outer In Sur- fer- size Recrystal- No. Steel (mm) (mm) No. (° C.) (min) PL (MPa) (MPa) Surface Wall face ence No. lization Remarks 31 B 323.9 25.0 4.5 650 264 19054 617 679 236 232 247 15 4.7 absent inventive ex. 32 B 323.9 25.0 4.5 650 294 19097 612 674 227 226 237 11 4.3 absent inventive ex. 33 B 323.9 25.0 4.3 650 315 19125 619 683 236 234 237 3 4.6 absent inventive ex. 34 B 323.9 25.0 4.3 630 204 18540 649 720 241 241 269 28 4.4 absent comparative ex. 35 B 323.9 25.0 4.5 630 219 18568 644 715 251 242 268 26 4.3 absent comparative ex. 36 B 323.9 25.0 4.6 630 234 18594 654 725 256 240 266 26 4.7 absent comparative ex. 37 B 323.9 25.0 4.3 630 249 18618 627 698 229 244 268 39 4.3 absent comparative ex. 38 B 323.9 25.0 4.3 630 264 18641 624 693 228 240 266 38 4.3 absent comparative ex. 39 B 323.9 25.0 4.6 630 294 18683 628 697 241 234 260 26 4.5 absent comparative ex. 40 B 323.9 25.0 4.3 600 204 17924 639 728 260 253 286 33 4.7 absent comparative ex. 41 B 323.9 25.0 4.5 655 105 18786 635 708 245 232 263 31 4.2 absent comparative ex. 42 B 323.9 25.0 4.5 650 105 18684 636 707 251 233 264 31 4.5 absent comparative ex. 43 C 323.9 25.0 4.6 650 105 18684 636 714 248 244 282 38 4.6 absent comparative ex. 44 C 323.9 25.0 4.6 650 185 18911 573 657 232 228 269 41 4.5 absent comparative ex. 45 D1 323.9 25.4 4.5 656 117 18849 478 561 178 178 205 27 4.5 absent comparative ex. 46 D2 323.9 25.4 4.3 650 130 18770 468 582 179 191 207 28 4.2 absent comparative ex. 47 E 406.4 38.1 4.3 600 204 17924 618 701 267 228 222 45 4.1 absent comparative ex. 48 E 406.4 38.1 4.6 630 204 18540 603 673 255 220 227 35 4.5 absent comparative ex. 49 E 406.4 38.1 4.5 630 234 18594 609 679 254 217 219 37 4.4 absent comparative ex. 50 E 406.4 38.1 4.3 630 264 18641 609 674 252 214 228 38 4.2 absent comparative ex. 51 E 406.4 38.1 4.3 630 294 18683 609 675 251 223 219 32 4.4 absent comparative ex. 52 E 406.4 38.1 4.3 650 204 18951 599 665 237 222 214 23 4.3 absent inventive ex. 53 E 406.4 38.1 4.5 650 234 19006 575 644 231 212 211 20 4.5 absent inventive ex. 54 E 406.4 38.1 4.5 650 264 19054 575 641 230 209 208 22 4.6 absent inventive ex. 55 E 406.4 38.1 4.5 650 294 19097 565 641 226 204 208 22 4.4 absent inventive ex. 56 E 406.4 38.1 4.6 660 204 19156 565 635 228 205 206 23 4.6 absent inventive ex. 57 E 406.4 38.1 4.3 660 234 19211 559 638 220 196 200 22 4.4 absent inventive ex. 58 F 323.9 25.0 4.5 650 200 18942 570 643 212 219 224 12 4.7 absent inventive ex. 59 G 323.9 25.0 5.4 650 200 18942 597 665 236 223 238 15 5.7 absent inventive ex. 60 I 273.1 25.0 5.5 650 200 18942 575 647 200 203 211 11 5.5 absent inventive ex. 61 J 323.9 25.0 4.7 665 149 19131 680 734 261 252 283 31 4.7 absent comparative ex. 62* K 323.9 25.0 6.8 665 234 19314 543 625 236 208 228 28 6.9 absent comparative ex. *62: in-line quenching at 950° C. + reheating at 950° C. and quenching + tempering

The seamless steel pipes that had been subjected to sizing rolling were heated by the supplementary heating furnace to 950° C., and quenching was then performed by the water-cooling apparatus where the pipes were cooled to room temperature at a cooling rate of 5° C./sec or higher.

After the quenching, the seamless steel pipes were tempered at the soaking temperatures and holding times shown in Tables 2 and 3. However, during the production of the steel of No. 62, after the above quenching was performed, before tempering, quenching was performed where the steel was reheated off-line to 950° C. and soaked for 20 minutes and then water-cooled.

The following evaluation tests were conducted on the seamless steel pipes produced in the above production process.

[Yield Strength and Tensile Strength Tests]

The yield strength of the seamless steel pipe of each number was investigated. More specifically, a No. 12 test specimen (with a width of 25 mm and a gauge length of 50 mm) as specified in JIS Z 2241 was taken out so that the longitudinal direction of the specimen for tensile testing was parallel to the longitudinal direction of the steel pipe (i.e. L direction). The test specimen that had been taken out was used to conduct a tensile test in compliance with JIS Z 2241 in the atmosphere at room temperature (25° C.), and the yield strength (YS) and tensile strength (TS) were determined. The yield strength was determined using a 0.5% total elongation method. The determined yield strength (in MPa) and tensile strength (in MPa) are shown in Tables 2 and 3. The columns labeled “YS” in Tables 2 and 3 have yield strength and the columns labeled “TS” have tensile strengths determined for the test specimens of the various test numbers.

[Surface Hardness Test]

Four test specimens were taken from the seamless steel pipe of each number, the specimens being displaced from each other by 90° along the pipe's circumference, and a Vickers hardness test in compliance with JIS Z 2244 was conducted on arbitrary three points on a transverse cross-section (i.e. cross-section perpendicular to the center axis) of each test specimen, the points being at 1 mm inwardly in the wall thickness direction from the inner surface. The force in the Vickers hardness tests, F, was 10 kgf (i.e. 98.07 N). The maximum among the values for the 12 points that had been obtained was used as the value of hardness “at 1 mm from the inner surface”.

Similarly, a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being at 1 mm inwardly in the wall thickness direction from the outer surface, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness “at 1 mm from the outer surface”. Further, a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being near the middle in the wall thickness, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness “in the wall”.

For the seamless steel pipe of each test number, the value of hardness “at 1 mm from the outer surface”, the value of hardness “at 1 mm from the inner surface” and the value of hardness “in the wall” are shown in Tables 2 and 3, in the columns labeled “Outer Surface”, “In Wall” and “Inner Surface”.

The largest value among the difference between the hardness “at 1 mm from the outer surface” and the hardness “in the wall”, the difference between the hardness “at 1 mm from the inner surface” and the hardness “in the wall”, and the difference between the hardness “at 1 mm from the outer surface” and the hardness “at 1 mm from the inner surface” (hereinafter referred to as “maximum difference in hardness”) is shown in the column labeled “Difference” in Tables 2 and 3.

[Observation of Microstructure]

A sample was taken from the seamless steel pipe of each number, the sample containing the inner surface, outer surface and middle in the wall thickness, and the microstructure was observed. More specifically, each sample was etched by a nital etching solution to cause the microstructure to appear, which was observed using optical microscopy.

The seamless steel pipe of each number had a microstructure having a main phase of tempered martensite or tempered bainite. However, in some seamless steel pipes, recrystallization of ferrite had occurred in a region of a depth of 1 mm or deeper from the surface. Whether recrystallization of ferrite occurred in a region of a depth of 1 mm or deeper from the surface is shown in the column labeled “Ferrite Recrystallization” in Tables 2 and 3.

The crystal grain size number of the prior austenite grains of the microstructure was measured by the following method: First, a test specimen was cut out from each steel pipe such that a cross section perpendicular to the length of the steel pipe as quenched (i.e. pipe forming direction) forms the observed surface, and was imbedded into a resin; then the Bechet-Beaujard method was used where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear, which were observed by optical microscopy (with a magnification of 200 times), and the prior austenite grain size number was measured in accordance with ASTM E112-10. Such grain size numbers are shown in the column “AsQ Prior γ grain size No.” in Tables 2 and 3.

Since the grain size number of prior austenite grains after tempering cannot be measured using picric acid saturated aqueous solution corrosion; in view of this, the number was measured with the help of EBSD. EBSD was performed by cutting out a test specimen such that a cross section perpendicular to the length of a tempered steel pipe forms the observed surface, finishing the observed surface by mirror polishing and electrolysis polishing, and an area of 500×500 pmt in the middle in the thickness of the steel pipe was observed. A detector for EBSD mounted on an FE-SEM (DigiViewIV from EDAX) was used. Based on the obtained crystal orientation data, analysis software (OIM Analysis ver. 6 from EDAX) was used to draw lines along the boundaries between crystal grains in misorientation angles of 15 to 51°, and the resulting line drawing was used to measure the prior austenite grain size number in accordance with ASTM E112-10. Such grain size numbers are shown in the column “QT Prior γ Grain Size No.” in Tables 2 and 3.

[Results of Investigation]

As shown in Tables 1 to 3, the seamless steel pipes of Nos. 19 to 33 and 52 to 60 had a chemical composition falling in the scope of the present invention and had a carbon equivalent Ceq not lower than 0.430% and lower than 0.500%. In these seamless steel pipes, recrystallization of ferrite did not occur in a region of a depth of 1 mm or deeper from the surface, and a structure was present having a main phase of tempered martensite or tempered bainite from the surface layer to the in-the-wall portions, and the crystal grain size number of the prior austenite grains was lower than 6.0. Further, these seamless steel pipes had Vickers hardness values “at 1 mm from the outer surface”, “at 1 mm from the inner surface” and “in the wall” that were not higher than 250 Hv and had a yield strength of 555 MPa or higher. These seamless steel pipes had a maximum difference in hardness of 25 Hv or lower.

The seamless steel pipes of Nos. 1 to 17 had a yield strength lower than 555 MPa. This is presumably because the carbon equivalent Ceq of steel A was too low.

In the seamless steel pipe of No. 18, recrystallization of ferrite occurred in a region of a depth of 1 mm or deeper from the surface. Consequently, the seamless steel pipe of No. 18 had a yield strength lower than 555 MPa. This is presumably because the Larson-Miller parameter PL of the seamless steel pipe of num No. ber 18 was too high.

The seamless steel pipes of Nos. 34 to 42 and 47 to 51 had a Vickers hardness value “at 1 mm from the outer surface”, “at 1 mm from the inner surface” or “in the wall” that was higher than 250 Hv. Further, these seamless steel pipes had a maximum difference in hardness higher than 25 By. This is presumably because the Larson-Miller parameters PL of the seamless steel pipes of Nos. 34 to 42 and 47 to 51 were too low.

The seamless steel pipes of Nos. 43 and 44 had a Vickers hardness “at 1 mm from the inner surface” higher than 250 Hv. This is presumably because the carbon equivalent Ceq of steel C was too high.

The seamless steel pipes of Nos. 45 and 46 had yield strengths lower than 555 MPa. This is presumably because the carbon equivalents Ceq of steels D1 and D2 were too low.

In the seamless steel pipe of No. 61, the Vickers hardness was higher than 250 Hv at all the measurement points. This is presumably because the carbon equivalent Ceq of steel J was too high.

The seamless steel pipe of No. 62 had a yield strength lower than 555 MPa. This is presumably because both in-line quenching and reheating-and-quenching were used, which produced too fine prior austenite grains, reducing hardenability and thus leading to insufficient strength.

FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel B. As shown in FIG. 4, the yield strength YS tended to decrease as the Larson-Miller parameter PL increased. Steel B provided a yield strength of 555 MPa or larger except for the seamless steel pipe of No. 18, in which the recrystallization of ferrite progressed.

FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A. Steel A did not provide a yield strength not lower than 555 MPa even though tempering conditions were adjusted. This is presumably because the carbon equivalent Ceq of steel A was too low.

FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B. As shown in FIG. 6, the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter PL increased. As shown in FIG. 6, when the Larson-Miller parameter PL was 18800 or higher, the hardnesses at the outer surface, in-the-wall portion and inner surface were 250 Hv or lower. On the other hand, when the Larson-Miller parameter PL was lower than 18800, the hardness at the outer surface, in-the-wall portion or inner surface was higher than 250 Hv.

FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A. In steel A, similar to steel B, the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter increased.

FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B. As shown in FIG. 8, when the Larson-Miller parameter PL was 18800 or higher, the maximum difference in hardness was not more than 25 Hv. The seamless steel pipe of No. 18 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1 mm or deeper from the surface.

FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A. As shown in FIG. 9, the relationship between Larson-Miller parameter PL and maximum difference in hardness in steel A exhibited similar tendencies. The seamless steel pipe of No. 3 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1 mm or deeper from the surface.

[Evaluation of Sour Resistance]

A sour resistance evaluation as described below (i.e. HIC resistance test, four-point bending test) was conducted on the seamless steel pipes of some of the numbers.

[HIC Resistance Test]

From each seamless steel pipe were taken out a test specimen containing the inner surface, a test specimen containing the middle in the wall thickness, and a test specimen containing the outer specimen. Each test specimen had a thickness of 20 mm and a width (along the circumference) of 20 mm, and a length of 100 mm. The MC resistance of each test specimen was evaluated in accordance with NACE (National Association of Corrosion Engineers) TM 0284-2011. The testing bath in which the test specimens were immersed was a 5% salt+0.5% acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24° C.

After 96 hours of immersion, ultrasonic inspection (UT) was conducted on the test specimens after being tested to determine the location of the largest crack, and the specimen was cut at this location. The cross-section at this time was a cross-section of thickness×width of the test specimen, i.e. perpendicular to the longitudinal direction of the steel pipe. The cut test specimen was used to determine the crack-length ratio CLR (=crack length (mm)/width of test specimen (mm)). The maximum value among the CLR values of the test specimen taken from each steel pipe was used as the crack-length ratio CLR for this test number.

Further, it was determined whether each test specimen after being tested had a blister (i.e. a swollen part due to a crack near the surface), and the number of blisters produced on the test specimen was counted. The maximum among the numbers of blisters on the test specimen taken from each steel pipe was used as the number of blisters for this test number.

[Four-Point Bending Test]

A stress of 95% of the actual yield strength (i.e. yield strength of the seamless steel pipe of each number) was applied to a test specimen containing the middle in the wall thickness of this seamless steel pipe using a four-point bending jig in accordance with ASTM G39. The test specimens to which stresses were applied were placed in a test bath. The test bath was a 5% salt+0.5% acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24° C. After 720 hours, it was visually determined whether there was a crack in the test specimens. If a plate material had no crack, it was determined that this material had good SSC resistance.

[Evaluation Results]

The results of sour resistance evaluation were shown in Table 4.

TABLE 4 Hv10kgf Four- (maximum for positions) HIC Point Outer In Inner Resistance Number of Bending No. PL YS (MPa) Surface Wall Surface Difference Test Blisters Test 10 18953 544 218 218 220 2 ◯ 0 ◯ 3 19314 509 197 195 162 35 ◯ 3 — 18 20132 503 209 193 170 39 ◯ 2 — 23 19131 621 238 240 239 2 ◯ 0 ◯ 33 19125 619 236 234 237 3 ◯ 0 ◯ 37 18618 627 229 244 268 39 CLR 1% 0 — 40 17924 639 260 253 286 33 CLR 2% 0 — 43 18684 636 248 244 282 38 CLR 2% — — 44 18911 573 232 228 269 41 CLR 2% — — 52 18951 599 237 222 214 23 ◯ 0 — 57 19211 559 220 198 200 22 ◯ 0 — 58 18942 570 212 219 224 12 ◯ 0 — 59 18942 597 236 223 238 15 ◯ 0 — 60 18942 575 200 203 211 11 ◯ 0 — In Table 4, “◯” in the columns labeled “HIC Resistance Test” and “Four-Point Bending Test” indicates that there was no crack in the relevant test. “ ” in the columns labeled “HIC Resistance Test” and “Four-Point Bending Test” indicates that the relevant test was not conducted.

As shown in Table 4, in the seamless steel pipes with a yield strength of 555 MPa or higher and Vickers hardness values “at 1 mm from the outer surface”, “at 1 mm from the inner surface” and “in the wall” not higher than 250 Hv, no crack occurs in both the HIC resistance test and four-point bending test, and a good sour resistance was provided in a reliable manner. On the other hand, the seamless steel pipes with Vickers hardness values “at 1 mm from the outer surface”, “at 1 mm from the inner surface” or “in the wall” higher than 250 Hv provided a poor sour resistance. These results prove a relationship between Vickers hardness and sour resistance.

Although embodiments of the present invention have been described, these embodiments are merely examples that may be used to carry out the present invention. Accordingly, the present invention is not limited to the above embodiments and the above embodiments can be modified as appropriate without departing from the spirit of the invention. 

1. A seamless steel pipe having a chemical composition of, in mass %, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities, where a carbon equivalent Ceq as defined by equation (1) below is not less than 0.430% and less than 0.500%, a main phase of a microstructure from a surface layer to an in-the-wall portion is tempered martensite or tempered bainite, a size of prior austenite grains in the microstructure is lower than 6.0 in crystal grain size number according to ASTM E112-10, a portion between a position at 1 mm from an inner surface and a position at 1 mm from an outer surface has a Vickers hardness of 250 Hv or lower, and a yield strength is 555 MPa or higher, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (1), where a symbol of each element in equation (1) is substituted by a content of this element in mass %.
 2. The seamless steel pipe according to claim 1, wherein the chemical composition contains, in mass %: Nb: 0.010 to 0.05%.
 3. The seamless steel pipe according to claim 1, wherein a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in a middle in a wall thickness, a difference between a Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface are each 25 Hv or lower.
 4. The seamless steel pipe according to claim 1, wherein: the seamless steel pipe is produced by quenching and tempering, and a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher: PL=(T+273)×(20+log(t))  (2), in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.
 5. A method of manufacturing a seamless steel pipe, comprising: preparing a raw material having a chemical composition of, in mass %, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities; hot working the raw material to produce a hollow shell; quenching the hollow shell by direct quenching or in-line quenching; and tempering the quenched hollow shell, no reheating-and-quenching is performed between the quenching and tempering, a carbon equivalent Ceq as defined by equation (3) below is not less than 0.430% and less than 0.500%, a Larson-Miller parameter PL as defined by equation (4) below is not less than 18800, Ceq=C+Mn/6+(Cr+Mo+V)/5±(Ni+Cu)/15  (3), and PL=(T+273)×(20+log(t))  (4), a symbol of each element in equation (3) is substituted by a content of this element in mass %, and in equation (4), T is a tempering temperature, and t is a holding period for this temperature, and T is in ° C., and t is in hours.
 6. The seamless steel pipe according to claim 2, wherein a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in a middle in a wall thickness, a difference between a Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface are each 25 Hv or lower.
 7. The seamless steel pipe according to claim 2, wherein: the seamless steel pipe is produced by quenching and tempering, and a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher: PL=(T+273)×(20+log(t))  (2), in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.
 8. The seamless steel pipe according to claim 3, wherein: the seamless steel pipe is produced by quenching and tempering, and a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher: PL=(T+273)×(20+log(t))  (2), in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.
 9. The seamless steel pipe according to claim 6, wherein: the seamless steel pipe is produced by quenching and tempering, and a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher: PL=(T+273)×(20+log(t))  (2), in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours. 